In U.S. Pat. No. 4,328,252, assigned to the assignee herein, the disclosure of which is incorporated herein by reference, there is described a process for the formation of protein fibres in which a protein isolate, known as protein micellar mass, sometimes referred to herein as "PMM", is injected into hot water having a temperature above 90.degree. C. through a plurality of openings.
The novel protein isolate is defined in U.S. Pat. No. 4,285,862, assigned to the assignee of this application, the disclosure of which is incorporated herein by reference, as a substantially undenatured protein isolate product containing at least about 90% by weight of protein (as determined by Kjeldahl nitrogen.times.6.25) and in the form of an amorphous protein mass which is formed by settling the solid phase from an aqueous dispersion of protein micelles consisting of homogeneous amphiphilic protein moieties and formed from at least one protein source material, the product having substantially no lipid content, substantially no lysinoalanine content and substantially the same lysine content as the storage protein in the source material.
Such novel protein isolate may be formed by the procedures defined in U.S. Pat. Nos. 4,169,090, 4,208,323, 4,296,026 and 4,307,014, assigned to the assignee herein, the disclosures of which are incorporated herein by reference. In these patents, there are described procedures for isolating protein from protein source materials by solubilizing the protein by contact of the protein source material with sodium chloride solution under critical pH and ionic strength conditions and diluting the protein solution with water to a lower ionic strength to cause the formation of the dispersion of protein micelles in the aqueous phase from which is settled as the amorphous protein micellar mass. The protein solution may be subjected to ultrafiltration prior to the dilution step and the settling may be enhanced by centrifugation.
The process of U.S. Pat. No. 4,169,090 involves solubilizing the protein in the vegetable protein source material at a temperature of about 15.degree. to 35.degree. C. using a food grade salt solution having a concentation of at least 0.2 molar ionic strength and a pH of 5.5 to 6.3 to form a protein solution, and diluting the protein solution to an ionic strength of less than 0.1 molar to cause formation of the dispersion.
The process of U.S. Pat. No. 4,208,323 involves solubilizing the protein in the vegetable protein source material at a temperature of about 15.degree. to about 35.degree. C. using a food grade salt solution having a concentration of at least 0.2 molar ionic strength and a pH of about 5 to about 6.8 to form a protein solution, increasing the protein concentration of the protein solution while maintaining the ionic strength thereof substantially constant, and diluting the concentrated protein solution to an ionic strength below about 0.2 molar to cause formation of the dispersion.
In the latter process, the food grade salt solution preferably has an ionic strength of about 0.2 to about 0.8 molar and a pH of about 5.3 to about 6.2. In addition, the protein concentration step is preferably effected by a membrane technique at a volume reduction factor about 1.1 to about 6.0, as determined by the ratio of volume of protein solution and the volume of concentated protein solution.
Further, the dilution of the concentrated protein solution is preferably effected by passing the concentrated protein solution into a body of water having a temperature below about 25.degree. C. and a volume sufficient to decrease the ionic strength of the concentrated protein solution to a value of about 0.06 to about 0.12 molar.
In one embodiment of the latter process, the food grade salt solution has a pH of about 5 to about 5.5 and the phosphorus content of the protein solution is decreased prior to the dilution step.
The food grade salt used in the above-described solubilization procedures usually is sodium chloride, although other salts, such as, potassium chloride or calcium chloride may be used.
As is set forth in U.S. Pat. No. 4,296,026, the purity of isolate which is obtained from soybeans may be improved by the presence of millimolar amounts of calcium chloride in the aqueous sodium chloride solution. As described therein, the protein is solubilized by contact with an aqueous sodium chloride solution having an ionic strength of at least about 0.2 molar and containing about 0.001 to about 0.01M calcium chloride and having a temperature of about 15.degree. to about 75.degree. C.
Further, as is set forth in U.S. Pat. No. 4,307,014, the yield of isolate which is obtained from soybeans may be improved by effecting the protein solubilization at a temperature of about 15.degree. to about 75.degree. C. using an aqueous food grade salt solution of ionic strength of at least 0.2M and a pH of about 5.6 to about 7.0, preferably about 6.0 to about 6.4, and then adjusting the pH of the protein solution to a pH of about 4.8 to about 5.4, preferably about 5.1 to about 5.3, prior to dilution of the pH-adjusted protein solution.
While the procedure of the aforementioned U.S. Pat. No. 4,238,252 produces protein fibres, the quality of the fibres obtained is sensitive to pH of the PMM and optimum fibre strength and elasticity are obtained only over a narrow range of about 5.6 to 5.7. Further, the range of fibre strength and elasticity, and hence texture variations which can be obtained, is quite narrow.